Method and system for temperature control in refrigerated storage spaces

ABSTRACT

The present invention relates to a method of controlling air temperature within a refrigerated storage space based on a temperature-error integral of supply air discharged into the storage space. Another aspect of the invention relates to a refrigerated storage space comprising a corresponding temperature control system.

The present invention relates to a method of controlling air temperaturewithin a refrigerated storage space based on a temperature-errorintegral of supply air discharged into the storage space. Another aspectof the invention relates to a refrigerated storage space comprising acorresponding temperature control system.

BACKGROUND OF THE INVENTION

It is important to maintain the temperature of perishable produce heldin refrigerated storage spaces at a desired or setpoint temperature. Thesetpoint temperature is chosen to keep the perishable produce such asmeat, vegetables and fruit, at correct temperature to avoid qualitydegradation. It is known in the art to apply temperature controlprotocols that selectively control operational states of heaters,cooling devices and internal fans such as evaporator fans of coolingunits, coupled to the refrigerated storage space, to maintain a setpointair temperature inside the refrigerated storage space. The refrigeratedstorage space may for example comprise a transport volume of arefrigerated container.

The typical cooling unit or refrigeration unit used in refrigeratedstorage spaces is based on the so-called vapour compressionrefrigeration cycle. This cycle comprises at least a compressor, acondenser, an expansion device, an evaporator and a capacity regulatingdevice. The compressor sucks refrigerant vapour from the evaporator andcompresses the refrigerant vapour which subsequently flows to thecondenser at high pressure. The condenser ejects its heat to a mediumoutside the refrigerated storage space while condensing the refrigerantvapour. The liquefied refrigerant then flows to the expansion device inwhich a refrigerant pressure drops. The low pressure refrigerant thenflows to the evaporator where the refrigerant evaporates whileextracting the required heat from the refrigerated storage space. Acapacity regulating device, which may comprise a suction modulatingvalve, controls the cooling capacity of the cooling unit. Coolingcapacity is the amount of heat absorbed by the evaporator per unit oftime. A typical characteristic of vapour compression refrigerationcycles is that their energy efficiency reduces in part-load operation,i.e. whenever the compressor continues to be driven or active while thecapacity regulating device reduces the cooling capacity.

EP 2 116 798 A1 discloses a refrigeration system which has an energysaving operation mode performing a first action in which a compressorand an internal fan are driven while the cooling capacity of anevaporator is regulated. In a second action, when the blow-off side airtemperature in a cold storage is kept at a set value in the firstaction, the cooling capacity of the evaporator is increased to lower theblow-off side air temperature to a lower limit temperature of a desiredtemperature range containing the set value and the compressor and theinternal fan are then stopped. In a third action in which, when theblow-off side air temperature after the second action rises to an upperlimit temperature of the desired temperature range, the first action isrestarted.

Despite various attempts to lower energy consumption of the coolingunits, substantial amounts of energy are still consumed in today'srefrigerated storage spaces. Consequently, there is a continuing needfor providing temperature control algorithms and systems with improvedenergy efficiency to reduce energy costs and reduce CO₂ emissions fromcooling or heating the refrigerated storage spaces.

In one embodiment, the present invention provides an improved energysaving methodology for controlling the respective operational states ormodes of heating units, cooling devices and internal fans of existingcooling units of refrigerated storage spaces. The method allowstemperature control systems of existing refrigerated storage spaces tobenefit from the invention without any need for hardware replacements ormodifications. The improved temperature control methodology mayadvantageously be implemented as embedded control software executed on amicroprocessor of a temperature control system associated with therefrigerated storage space to improve energy efficiency of existingheating units, cooling units and internal fans or circulation fans.Consequently, the present invention may conveniently, but notexclusively, be implemented by a software update of existing embeddedcontrol software or program code of the temperature control system.

SUMMARY OF INVENTION

A first aspect of the invention relates to a method of controlling airtemperature of a refrigerated storage space, the method comprising stepsof:

-   -   circulating return air from the refrigerated storage space        through a cooling unit to provide a flow of cooled supply air at        a supply air temperature,    -   discharging the supply air into the refrigerated storage space        to control the air temperature inside the refrigerated storage        space,    -   measuring the supply air temperature by a first temperature        sensor,    -   computing a temperature-error integral of the supply air based        on a difference over time between the supply air temperature and        a reference temperature,    -   adjusting the supply air temperature based on the        temperature-error integral such that a time average of the        supply air temperature substantially equals the reference        temperature.

The time average of the supply air temperature preferably deviates withless than +/−0.5° C., or more preferably with less than +/−0.2° C., fromthe reference temperature under steady-state operation of a temperaturecontrol system or algorithm comprising the present methodology ofcontrolling air temperature of the refrigerated storage space.

The refrigerated storage space may comprise various types of stationaryor transportable refrigerated compartments or spaces such as spaces ofhousehold freezers and refrigerators, cold storage houses andrefrigerated containers.

In accordance with the present invention, the temperature of the cooledsupply air or supply air is adjusted based on the temperature-errorintegral, which is an integral over the difference between the supplyair temperature and the predetermined reference temperature. The use ofthe supply air temperature-error integral allows wider fluctuation ofinstantaneous supply air temperature while yet providing accuratecontrol of average supply air temperature over time. In contrast,traditional temperature control algorithms or schemes seek to maintainthe instantaneous supply air temperature at setpoint temperature,possibly within upper and lower temperature limits. The widerfluctuations of the instantaneous supply air temperature afforded by thepresent temperature control methodology or scheme enables exclusivelyswitching operational states of the cooling unit or device between ONand OFF while maintaining accurate control over the time-averaged supplyair temperature. The exclusive switching of operational states of thecooling unit between ON and OFF, while maintaining accurate control overthe time-averaged supply air temperature such as within theabove-mentioned temperature deviations, may be provided by pure integralcontrol (1-control) based adjustment of the supply air temperature basedon a current value of an error signal derived from, or constituted by,the temperature-error integral.

In the present specification, the ON state of the cooling unit or devicemeans it operates at, or close to, its maximum capacity, i.e. about 100%capacity such as at more than 85% of its maximum cooling capacity, evenmore preferably above 90 or 95% of its maximum cooling capacity. The ONoperation at, or close to, the maximum capacity of the cooling unitavoids energy inefficient part-load cooling. The ON state is furthermorepreferably placed at a percentage of the maximum cooling capacity whereoperation is highly efficient. This percentage may vary between specificcooling units but typically lies in the above-mentioned range between85-100% of the maximum cooling capacity.

Despite the allowed wider fluctuations of the instantaneous supply airtemperature, average supply air temperature is accurately controlled.The temperature of the commodity load, which typically comprisesperishable produce, situated within the transport volume of thecontainer is maintained within tight limits or bounds due to thermalinertia of the commodity load despite the wider fluctuations of theinstantaneous supply air temperature.

The reference temperature may comprise, or be set to, an adjustedsetpoint temperature or a setpoint temperature of the refrigeratedstorage space. The adjusted setpoint temperature is preferably derivedfrom the setpoint temperature and possibly one or more additionaltemperature variables of the temperature control algorithm.

The present methodology for air temperature control within the transportvolume preferably comprises controlling both the cooling unit and aheating unit. Accordingly, in one embodiment, the present methodologycomprises a further step of:

-   -   circulating the return air from the refrigerated storage space        through the heating unit to provide a flow of heated supply air        at the supply air temperature. This embodiment enables the        present temperature control methodology to provide a flow of        both cooled supply air and heated supply air, depending on the        circumstances, to maintain the commodity load at a desired        temperature across a wide range of external environmental        temperatures outside the refrigerated storage space.

In the present specification, the ON state of the heating unit or devicemeans it operates at, or close to, its maximum capacity, i.e. about 100%capacity such as at more than 85% of its maximum heating capacity, evenmore preferably above 90 or 95% of its maximum heating capacity.Furthermore, the ON state is preferably placed at a percentage of themaximum heating capacity where operation is highly efficient. Thispercentage may vary between specific types of heating units.

The computation of the temperature-error integral of the supply air ispreferably performed by the temperature control system operativelycoupled to the refrigerated storage space. The temperature controlsystem may reside in a dedicated cooling unit mounted to a wall sectionof the refrigerated storage space. Alternatively, certain parts of thetemperature control system may be situated remotely and coupled tocontrol operation of the cooling unit, heating unit etc through a wiredor wireless communications interface.

The temperature control system may comprise a microprocessor operatingaccording to a set of embedded program instructions or embeddedsoftware. The embedded software may be adapted to receive and processsupply air temperature data provided by the first temperature sensor anddetermine appropriate control actions of the cooling unit and/or heatingunit to adjust the supply air temperature in a desired direction toreach the desired air temperature inside the refrigerated storage space.The temperature-error integral is preferably computed at regular timeintervals such as time intervals smaller than 1 minute, preferablysmaller than 10 seconds such as about every second.

In accordance with a preferred embodiment of the invention, the methodof controlling air temperature comprises a step of:

-   -   adjusting the reference temperature as a function of the        setpoint temperature and a temperature of the return air        measured by a return air temperature sensor or second        temperature sensor. Hereafter this adjusted reference        temperature or adjusted setpoint temperature is denoted by        T_(set) _(—) _(quest). The setpoint temperature is normally        identical to a displayed setpoint temperature of the        refrigeration unit of the refrigerated storage space. By        adjusting the reference temperature as function of the setpoint        temperature and the temperature of the return air, the inventors        have demonstrated improved quality preservation of the        perishable produce by improved produce temperature control. This        has been achieved because average produce temperature within the        refrigerated transport volume typically lies somewhere        in-between the supply air temperature and a few degrees above        the return air temperature due to temperature gradients within        the transport volume.

In one embodiment, the adjustment of the reference temperature is madesuch that the average of the supply air temperature and the return airtemperature substantially equals, preferably within +/−0.2 degree C.,the setpoint temperature unless upper and lower limits for the referencetemperature prohibit this. The average produce temperature is therebymaintained close to the setpoint temperature which represents thedesired commodity load temperature.

The method of controlling air temperature according to the inventionpreferably comprises a step of controlling respective operational statesof the heating unit or the cooling unit or both to adjust the supply airtemperature. As previously mentioned, the respective operational statesof the heating and cooling units are preferably switched between ON andOFF. In one embodiment, the method comprises a step of exclusivelyswitching operational states of the cooling unit between ON and OFF tomake the adjustment of the supply air temperature. This embodiment isparticularly advantageous since it avoids time periods of inefficientpart-load refrigeration, leading to markedly improved energy efficiency.

In the present specification, the ON state of a heating or cooling unitmeans the unit in question operates at, or close to, its maximumcapacity. In certain embodiments, the cooling unit and/or the heatingunit may only possess a fixed number of discrete operational states suchas two, three or four etc. In other embodiments, the operational stateof the cooling unit and/or the heating unit may be continuously variablebetween for example 0% (OFF) and the maximum capacity.

The cooling unit may comprise a compressor coupled to an evaporator witha capacity regulating device regulating the cooling capacity of thecooling unit. In one such embodiment the compressor is coupled to theevaporator via the capacity regulating device, such as a suction valve,mounted on a fluid connection between the compressor and the evaporator.In such an embodiment, the present methodology may comprise a step of:

-   -   controlling a refrigerant flow rate between the evaporator and        the compressor by a capacity regulating device to adjust a        cooling capacity of the evaporator. In a particularly        advantageous embodiment, the operational state of the capacity        regulating device is such that the refrigeration capacity is        substantially maximized during time periods, preferably all time        periods, where the compressor is ON, i.e. operating at, or close        to, its maximum capacity.

According to yet another preferred embodiment, the temperature-errorintegral of the supply air is controlled to stay within upper and lowerbounds or limits as defined by first and second integral errorthresholds, respectively. The method comprising further steps of:

-   -   comparing the temperature-error integral of the supply air with        the integral first integral error threshold and the second        integral error threshold,    -   changing an operational state of at least one of the cooling        unit, the heating unit and the internal fans if the        temperature-error integral exceeds the first integral error        threshold or falls below the second integral error threshold.

The operational state of the cooling unit is controlled such that thecooling unit preferably is activated if the first integral errorthreshold is exceeded indicating the average supply air temperature overa preceding time period, such as a time period of a current cycle, wastoo high. Likewise, the heating unit is preferably activated iftemperature-error integral falls below the second integral errorthreshold indicating that average supply air temperature over the pasttime period was too low.

Experimental investigations by the present inventors indicate that thefirst integral error threshold may be set to a value between 50 and 200°C.*minutes (the unit being ° C. times minutes) and the second integralerror threshold set to a value between −100 and −10° C.*minutes fortypical refrigerated storage spaces. A difference between the firstintegral error threshold and the second integral error threshold may beset to a value between 20 and 200° C.*minutes to have sufficient, butnot too much, bandwidth or distance between cooling and heatingoperation.

The methodology of controlling air temperature-error integral to staybetween the first and second integral error thresholds may comprisefurther steps of:

-   -   setting the operational state of the cooling unit to active for        at least a first minimum time period if the temperature-error        integral exceeds the first integral error threshold,    -   setting the operational state of the heating unit to active for        at least a second minimum time period if the temperature-error        integral is smaller than the second integral error threshold.

The active operational state of the cooling unit is preferably ON at alltimes and the active operational state of the heating unit preferably ONat all times as well. Each of the first and second minimum time periodsmay be set to a value between 1 and 10 minutes such as around 5 minutesto avoid unnecessary wear and tear of the compressor and electricalcontactors.

According to another preferred embodiment, the method of controlling airtemperature comprises a step of:

-   -   limiting the reference temperature to a lower temperature limit        dependent on the setpoint temperature. Optionally, the reference        temperature may also be limited to an upper temperature limit.        Limiting how low the reference temperature can drop relative to        the setpoint temperature is very helpful in avoiding chilling or        freezing damage to the commodity load caused by possible        prolonged drops of the supply air temperature. For some setpoint        temperatures the lower temperature limit is to be substantially        equal to the setpoint temperature. For other setpoint        temperatures the lower temperature limit is set to a value        between 0.1 and 2.0° C. below the setpoint temperature.

In another embodiment, the method of controlling air temperaturecomprises steps of, during circulation periods, comparing thetemperature-error integral to a first heating threshold and maintainingthe internal fans at a first preset speed if the temperature-errorintegral is below the first heating threshold. In the presentspecification, the term “circulation period” means time periods wherethe operational state of the heating unit is OFF and the operationalstate of the cooling unit is OFF. The internal fans may be configured tooperate at a number of discrete preset speed settings such as OFF, Lowand High with a predefined speed ratio between the Low and High speedsettings such as a ratio of 2 or 3 or more. The OFF, Low and High speedsettings may for example correspond to air flow rates of 0, 3000 and6000 m³ per hour, respectively. The first preset speed may in thissituation be either Low or High. By forcing the internal fans to run atthe first preset speed if the temperature-error integral lies below thefirst heating threshold, the temperature control system or algorithmonly activates the heating unit if fan energy does not suffice to supplyrequired heat energy. Fan energy is an advantageous heating source dueto its double effect contributing with both heating of the supply airand mixing of the air inside the refrigerated storage space.

In the present specification each of the discussed fan speed settings ofthe internal fan or fans may be provided by joined operation of allinternal fans present in the refrigerated storage space. Different fanspeed settings may be achieved by changing the actual speed of one orseveral individual fan(s) or by turning a certain number of fans ON orOFF.

This embodiment may comprise a further step of:

-   -   comparing the temperature-error integral to a second heating        threshold smaller than the first heating threshold,    -   maintain the internal fans at a second preset speed if the        temperature-error integral is below the second heating        threshold; the second preset speed is higher than the first        preset speed. In the above described situation with a set of        discrete fan speeds, the second preset speed may be High if the        first preset speed is Low. In other embodiments, the internal        fans speed may be adjustable over a continuous speed range and        the second preset speed set to any speed higher than the first        preset speed. One advantage of using multiple heating thresholds        is a maximization of the ratio air flow divided by energy input        because physics dictate that air flow generated by a fan is a        linear function of the cube of its power draw.

In another embodiment, the present methodology comprises further stepsof:

-   -   comparing a duration of a previous circulation period with a        circulation time threshold t_(ct),    -   maintain the speed of the internal fans at a maximum speed        during a current circulation period if the previous circulation        period was smaller than the circulation time threshold t_(ct).        This embodiment ensures effective air circulation within the        refrigerated storage space during the current circulation period        if a previous circulation period was relatively short, i.e.        below the circulation time threshold t_(ct). Such relatively        short previous circulation periods may indicate a relatively        large net heat load on the refrigerated storage space which        makes it important to provide high air circulation to limit or        decrease air temperature distribution differences within the        refrigerated storage space. In the present specification, the        “net heat load” means the sum of heat ingress into the        refrigerated storage space and autonomous heat production of the        commodity load within the refrigerated storage space.

According to a further refinement of the above-mentioned embodiment, thecirculation time threshold t_(ct) depends on a change of thetemperature-error integral during the previous circulation period.

A number of embodiments of the present invention are advantageouslyconfigured to control relative humidity (RH) of the air insiderefrigerated storage spaces in addition to controlling the airtemperature. In one embodiment the methodology comprises further stepsof:

-   -   simultaneously heating and cooling the supply air at a first        speed setting of internal fans when a measured relative humidity        (RH) of the air inside the transport volume is higher than a        first humidity threshold derived from the setpoint value of the        relative humidity (RH_(set)),    -   setting a second fan speed of the internal fans during        circulation periods when a measured relative humidity (RH) of        the air is higher than a second humidity threshold derived from        the setpoint value of the relative humidity;    -   wherein the first fan speed setting is a lower fan speed than        the second fan speed setting. The first fan speed setting may be        Low and the second fan speed setting High. The lower speed of        the first fan speed setting is preferred because it increases a        dehumidification capacity of a refrigeration unit controlled by        the temperature control system.

In another embodiment, the above embodiment with switching between thefirst and second fan speed settings may comprise further steps of:

-   -   setting the heating unit to an ON state during circulation        periods when a measured relative humidity of the air is higher        than a third humidity threshold derived from the setpoint value        of the relative humidity. By increasing heat production during        circulation periods as outlined above, the duration of the        circulation period is shortened by evoking earlier cooling and        therefore also earlier dehumidification.

A second aspect of the invention relates to a refrigerated storage spacecomprising a refrigerated volume for housing a commodity load. A coolingunit is configured to receive return air from the refrigerated volumeand generate a flow of cooled supply air at a supply air temperature andan air flow passage is coupled to the refrigerated volume to dischargethe supply air therein and control air temperature within therefrigerated volume. A first temperature sensor is adapted to measuringthe supply air temperature. A temperature control system is adapted tocomputing a temperature-error integral of the supply air based on adifference over time between the supply air temperature and a referencetemperature. The temperature control system is additionally adapted toadjusting the supply air temperature based on the temperature-errorintegral such that a time average of the supply air temperaturesubstantially equals the reference temperature.

The return air from the refrigerated volume may be conveyed to thecooling unit or the heating unit through a second air flow passage. Thesecond air flow passage may in certain embodiments be located close to aceiling portion of the refrigerated storage space. A second or returnair temperature sensor may be provided for determining the return airtemperature. As previously mentioned, the computation of thetemperature-error integral of the supply air is preferably performed bya temperature control algorithm executed by the temperature controlsystem operatively coupled to the refrigerated storage space. Thetemperature control system may comprise a microprocessor operatingaccording to a set of embedded program instructions or embedded softwareto execute the temperature control algorithm. Alternatively, thetemperature control system may comprise dedicated computation hardwaresuch as programmable logic or hardwired arithmetic and logic circuitblocks configured to execute the required computational steps of thetemperature control algorithm.

As previously mentioned, the refrigerated storage space preferablycomprises a heating unit configured to supply heated supply air at thesupply air temperature by circulating the return air from therefrigerated storage space. The temperature control system may beadapted to controlling respective operational states of the heating unitor the cooling unit or both to adjust the supply air temperature.

To maximize cooling energy efficiency of the cooling unit, thetemperature control system may be adapted to exclusively switchingoperational states of the cooling unit between ON and OFF states to makethe adjustment of the supply air temperature. This may be achieved byswitching operational states of a compressor of the cooling unit betweenexclusively ON and OFF states so as to avoid energy inefficientpart-load operation of the compressor.

According to an advantageous embodiment, the temperature control systemof the refrigerated storage space is adapted to adjust the referencetemperature as a function of a setpoint temperature and a temperature ofthe return air measured by a return air temperature sensor. In thisembodiment, the temperature control system is preferably further adaptedto adjust the reference temperature such that the average of the supplyair temperature and the return air temperature substantially equals thesetpoint temperature. By controlling the average of the supply airtemperature and the return air temperature to the setpoint temperature,improved produce quality preservation can be achieved due to improvedproduce temperature control. The present inventors have exploited thefact that average produce temperature in the refrigerated transportvolume is closer to the mean of the supply and return air temperaturethan to the supply air temperature.

The temperature control system of the present refrigerated storage spacemay naturally be further adapted or refined to comprise any of thepreviously described functions or features in accordance with the firstaspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will be described in more detailin connection with the appended drawings, in which:

FIG. 1 is a simplified side cross-sectional view of a refrigeratedcontainer in accordance with a preferred embodiment of the invention,

FIG. 2 is a state diagram illustrating respective operational modes of acooling unit, a heating unit and internal fans as function of atemperature-error integral of supply air discharged into a transportvolume of the refrigerated container in accordance with the preferredembodiment of the invention,

FIGS. 3 and 4 are flow charts illustrating program steps executed by amicroprocessor-implemented temperature control algorithm of atemperature control system of the refrigerated container in accordancewith a preferred embodiment of the invention,

FIG. 5 illustrates logic rules applied by the temperature controlalgorithm for controlling internal fans speed,

FIG. 6 comprises a series of graphs illustrating experimentally recordedvalues of various key variables of the temperature control algorithmunder high net heat load conditions in accordance with the preferredembodiment of the invention,

FIG. 7 comprises a series of graphs illustrating the same variables asFIG. 6 but under low net heat load conditions,

FIG. 8 comprises a series of graphs illustrating the same variables asFIG. 6 but under conditions where heating is required; and

FIG. 9 is a graph illustrating experimentally recorded values of producetemperature versus supply air temperature.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, the refrigerated container 1 comprises a frontal sectionhaving a refrigeration unit 40 housing multiple, usually two or three,variable speed internal fans 10 (depicted schematically as a singlefan), heating unit or element 20 and a cooling unit 15. A load or cargosection 30 of the refrigerated container 1 comprises a commodity loadcomprising a plurality of stackable transport boxes 35 arranged within atransport volume 45 such as to leave appropriate clearance at a ceilingand a floor structure for air flow passages above and beneath thecommodity load. The frontal section comprises a refrigeration unit 40comprising return air temperature sensor 5 adapted to measure atemperature of a return air flow 50 comprising air that has beencirculated through the transport volume 45. Another temperature sensor25 is adapted to measuring a supply air temperature of heated or cooledsupply air 55 discharged into the transport volume 45 through an airflow passage. A temperature control system (not illustrated) comprises aprogrammed microprocessor which controls respective operational statesof the variable speed internal fans 10, the heating unit 20 and thecooling unit 15 in accordance with a temperature control algorithmdefined by a set of microprocessor program instructions. The temperaturecontrol system additionally comprises a user interface, for example aLCD display, where an operator or ship technician can enter or modifycertain parameter values of the temperature control algorithm such as asetpoint temperature of the refrigerated container 1. The operation ofthe temperature control algorithm is explained in detail below withreference to FIG. 2 and FIGS. 5 & 6.

The state diagram 200 of FIG. 2 schematically illustrates how switchingbetween control modes or states of the present temperature controlalgorithm is performed in a preferred embodiment as function of atemperature-error integral of the supply air discharged into a transportvolume of the refrigerated container. Arrow 202 points in direction ofincreasing values of the temperature-error integral (TEI).

The state or domain diagram 200 comprises a number of temperature-errorintegral thresholds or limits in-between individual control states 204,206, 208, 210 and 212. A first threshold, TEI_heat_stage_(—)3_lim,constitutes a lower integral error threshold such that the heating unitis switched ON if a current value of the temperature-error integralfalls below this threshold. In the upper portion of the state diagram200 in-between control states 204 and 206, a first threshold,TEI_max_cool, constitutes a first or upper integral error threshold suchthat the cooling unit is switched ON if a current value of thetemperature-error integral exceeds this upper threshold.

Three intermediate states, where the operational states of both thecooling unit and the heating unit are OFF, are located in abutmentin-between the upper control state 204, cooling, and the lowermostcontrol state 212, heating. These three intermediate states comprise twoheating states, control states 208 and 210, and a circulation state 206.In the control states 208 and 210, the heating unit resides inoperational state OFF and the internal fans are exploited to both supplyheat to the supply air and add circulation to the air inside thetransport volume. In the control state 208 the internal fans are set inthe Low speed operational state while the internal fans speed is set toHigh speed in the control state 210 reflecting a requirement for higherheat production due to the decreasing value of the TEI as indicated bythe arrow 202.

In the circulation control state 206, the internal fans may be switchedbetween three different operational states having different fan speedssuch as Off, Low and High. In this embodiment internal fans speed isswitched between High, Low and Off during circulation periods. The speedof the internal fans is maintained at a maximum or High speed during acurrent circulation period if the previous circulation period wassmaller than the circulation time threshold t_(ct). Otherwise the fanspeed is kept at Low speed at the start of the circulation period andsubsequently enters a fan speed cycling program during which:

-   -   the fan speed is reduced by one step such as from High to Low        speed after maintaining the fan speed for its predetermined        maximum duration setting. An additional condition for switching        from Low to Off is that the previous circulation fan speed was        High.    -   The fan speed is increased by one step after maintaining the fan        speed for its predetermined maximum duration setting. An        additional condition for making a change from Low to High speed        is that the previous circulation period fan speed was Off.    -   The fan speed is reduced by one step if the fan speed was        increased a preset time period ago such as 10 or 5 minutes,        while changes to the return air temperature Tret in that preset        time period fell within predetermined upper and lower limits or        bounds.    -   The fan speed is increased by one step if the change of the        return air temperature Tret since the start of the current fan        speed setting fell outside predetermined upper and lower limits        or bounds.

This above set of rules implies that the fan speed setting of theinternal fans always changes speed setting one step at a time, so neverfrom Off to High or vice versa. The above-described algorithm or rulesfor switching between fans speed setting during the circulation periodis schematically illustrated by FIG. 5.

In the present embodiment, the setting of the heating thresholds betweenthe individual states 204, 206, 208, 210 and 212 are:

TEI_max_cool=90° C.*min,TEI_heat_stage_(—)1_lim=0° C.*min,TEI_heat_stage_(—)2_lim=−10° C.*min,TEI_heat_stage_(—)3_lim=−30° C.*min.

The variables used in describing the present embodiments of thetemperature control algorithm are defined below in Table 1.

TABLE 1 Definition of variables of the temperature control algorithm.Variable/acronym Description Tset_quest [° C.] Reference temperature towhich cycle-averaged supply air temperature is controlled cycle_length[min] A length of a current cycle Cycle Period of time used in thetemperature control algorithm. In usual cooling operation it is the timeelapsed between two consecutive cooling compressor starts. Tret_avg [°C.] Average return air temperature during last completed cycle TEI [°C.*min] Supply air temperature-error integral, i.e. integral over Tsupminus Tset_quest. Tsup [° C.] Supply air temperature Tret [° C.] Returnair temperature Tset [° C.] Current temperature setpoint ΔTset_quest_minTset_quest_min = Tset + ΔTset_quest_min is a lower bound on [° C.]Tset_quest. This bound may esp. be hit during high heat load.ΔTset_quest_max Tset_quest_max = Tset + ΔTset_quest_max [° C.] is anupper bound [° C.] on Tset_quest. This bound may esp. be hit duringlarge negative heat load, like may occur when Tset = +30° C. in Canadianwinters. TEI_max_cool [° C.*min] See FIG. 2. Cooling switches ON if  ∫_(t₀)^(t)(T_(sup)(τ) − Tset_quest(τ)) d τ > TEI_max_cool. Itwill only switch off again when min. on-time has passed by and∫_(t₀)^(t)(T_(sup)(τ) − Tset_quest(τ)) d τ ≤ TEI_max_cool.TEI_heat_stage3_lim [° C.*min] See FIG. 2. Heating switches on when  ∫_(t₀)^(t)(T_(sup)(τ) − Tset_quest(τ)) d τ < TEI_heat_stage3_lim.It will only switch off again when min. on-time has passed by and∫_(t₀)^(t)(T_(sup)(τ) − Tset_quest(τ)) d τ ≥ TEI_heat_stage3_lim.TEI_max [° C.*min] To avoid integral wind-up the value of TEI is limitedto the range TEI_min ≦ TEI ≦ TEI_max. TEI_min [° C.*min] To avoidintegral wind-up the value of TEI is limited to the range TEI_min ≦ TEI≦ TEI_max. t_(ct) [min] circulation time threshold.

In the present embodiment of the invention, the calculated supply airtemperature-error integral TEI is bounded to the interval [TEI_min,TEI_max]. TEI(t) [° C.*min] is calculated in time discrete format by:

TEI_now=(Tsup−Tset_quest)*ts.

TEI=max(TEI_min,min(TEI_max,TEI+TEI_now));

where ts is a sampling interval or time period.

Experiments have revealed that the sampling time period, ts, preferablyshould be less than 10 seconds such as about 1 second.

A starting value for TEI is determined anytime the temperature controlalgorithm has not been operating. This condition will occur followingalgorithm power-up. In these cases, the internal fans are first run atHigh speed for 15 seconds and then an initial value of TEI is calculatedusing:

TEI=max(TEI_min,min(TEI_max, 40*(Tret—Tset_quest)+30); wherein

the anti-integral windup precautions max( . . . , min( . . . , . . . ))avoid the integral from getting excessively large.

In the present embodiment of the invention the reference temperature isan adjusted setpoint temperature Tset_quest. The setpoint adjustment ismade such that an average of the supply air temperature Tsup and ameasured return air temperature Tret substantially equals a setpointtemperature Tset. More specifically, in the present temperature controlalgorithm, a cycle-averaged supply air temperature Tsup is controlled tothe adjusted setpoint temperature Tset_quest.

The cycle is defined as a period of time starting at an end of aprevious cycle and ending when one of the following three conditionsapplies:

-   -   1. Cooling unit switches ON,    -   2. Heating unit switches OFF,    -   3. Last cycle ended more than 1 hour ago

During each cycle, the cycle length and the average of the return airtemperature are updated at regular time intervals. These values are usedto calculate Tset_quest at the start of the next cycle. At the start ofthe first cycle after power-up Tset_quest is set to Tset. Following thisinitialization, Tset_quest is calculated at the beginning of eachsubsequent cycle according to the equations below:

Tset_quest_new=(1−0.2*cycle_length/60)*Tset_quest+0.2*cycle_length/60*(Tset−(Tret_avg−Tset));

Tset_quest=max(Tset+ΔTset_quest_min;min(Tset+ΔTset_quest_max;Tset_quest_new)).

Please refer to the definition of the above variables in Table 1.

The flowchart extending across FIGS. 3 and 4 provides a simplifiedsummary of the above-described operation of the present temperaturecontrol algorithm or algorithm during each call to the temperaturecontrol algorithm. The algorithm starts in step 301 and proceeds to step303 wherein the value of the temperature-error integral TEI is updatedusing the current supply air temperature and Tset_quest. In step 303also the average return air temperature Tret_avg is updated using acurrent return air temperature.

In step 305 the algorithm checks whether all respective minimum ON orOFF time periods associated with the heating unit, cooling unit andinternal fans speed have expired. Suitable minimum ON and OFF times aregenerally in the range between 1 and 10 minutes such as about 5 minutesbut may be adjusted based on specific characteristics of variouscomponents of the refrigeration unit.

If these minimum ON or OFF time periods have not expired, the algorithmproceeds to step 319 and maintains current operational states of theheating unit, cooling unit and internal fans. Thereafter, the algorithmproceeds to step 401. On the other hand if all minimum ON or OFF timeperiods have expired, the algorithm proceeds to step 307 and testswhether the current value of the temperature-error integral TEI exceedsthe upper bound or first error threshold TEI_max_cool of thetemperature-error integral. If the current value of thetemperature-error integral TEI exceeds the upper bound it indicates thatthe average supply air temperature through the current cycle is gettingtoo high. Therefore, the operational state of the cooling unit isswitched to ON in step 321 and the state of the heating unit is set toOff. The internal fans speed is also switched to, or maintained at, Highspeed setting. The High speed setting during cooling is advantageous forseveral reasons, one of them being less dehumidification and thereforeless weight loss to the commodity load. After step 321, the algorithmproceeds to step 401.

On the other hand if the current value of the temperature-error integralTEI is smaller than the upper bound, the algorithm either switches theoperational state of the cooling unit to OFF, or maintains an alreadyexisting OFF state, in step 309. The algorithm proceeds to step 311 andtests whether the current value of the temperature-error integral TEI issmaller than the lower bound or second integral error thresholdTEI_heat_stage3_lim of the temperature-error integral. If the currentvalue of the temperature-error integral TEI is smaller than the lowerbound it indicates that the average supply air temperature through thecurrent cycle is getting too low. Therefore, the algorithm proceeds tostep 323 and switches the operational state of the heating unit to ON.The internal fans speed is also switched to, or maintained at, the Highsetting so as to add heat and homogenise or minimize temperaturevariations within the transport volume. After step 323, the algorithmproceeds to step 401 with the effect described below.

On the other hand if the current value of the temperature-error integralTEI is larger than the lower bound, the algorithm proceeds to step 313and sets or switches the operational state of the heating unit to Offand proceeds to step 315. In step 315 it is evaluated whether theprevious circulation period was shorter than the circulation timethreshold t_(ct). If true (Y), the algorithm proceeds to step 325 andsets the internal fans speed to High, after which the algorithm proceedsto step 401.

On the other hand if in step 315 it is evaluated that the previouscirculation period was not (N) shorter than the circulation timethreshold t_(ct), the algorithm leaves the fan speed to the circulationperiod's fan cycling program in step 317. The algorithm proceeds to step317 and determines the appropriate internal fans speed or state (i.e.OFF, Low, High) by applying the logic rules governing internal fansstate during circulation periods as outlined above in connection withthe description of FIG. 2. After setting the appropriate internal fansspeed, the algorithm proceeds to step 401 (refer to FIG. 4).

In step 401, the algorithm checks or determines whether or not thecurrent cycle has been completed. This is done by evaluating the threelogic rules or conditions outlined before and determining if one ofthese conditions applies. The algorithm proceeds to step 405 if thealgorithm determines that the cycle has been completed and computes anupdated value of the adjusted setpoint temperature Tset_quest based onsetpoint temperature Tset and the average return air temperatureTret_avg.

The value of the average return air temperature Tret_avg is thereafterskipped and preparations are made for calculation of the average returnair temperature in the upcoming cycle. The algorithm proceeds to itsending in step 407.

If the algorithm in step 401 determines that the current cycle has notbeen completed, a current value of the adjusted setpoint temperatureTset_quest is maintained in step 403. After that the algorithm proceedsto its ending in step 407. Thereafter the algorithm proceeds to await anext call of a control algorithm at step 301. The next call willtypically occur after a certain delay period, i.e. the sampling timeinterval minus computation time.

FIG. 6 is series of graphs 601, 603 and 605 illustrating experimentallyrecorded values of selected variables of the above-described temperaturecontrol algorithm under high net heat load conditions of therefrigerated container.

Graph 601 shows temperature values in ° C. on the y-axis for theadjusted setpoint temperature Tset_quest (long dotted line), the returnair temperature Tret (short dotted line) and the supply air temperatureTsup (full line). The x-axis unit is time in minutes.

Graph 603 shows corresponding (to graph 601) values of the computedtemperature-error integral (TEI) in units of ° C.*min on the y-axis andtime in minutes on the y-axis. The full line represents values of theTEI and the horizontal dotted line represents a value of 90° C.*min fora first or upper integral threshold error TEI_max_cool of thetemperature-error integral.

Finally, graph 605 shows corresponding (to graphs 601 and 603) operatingstates of the cooling unit, heating unit and internal fans where thestates are indicated on the y-axis. The respective ON states of thecooling unit and heating unit are indicated by the value “1” and OFFstates as the value “0”. For the internal fans, the High setting orstate (maximum fan speed) is indicated as “2”, the Low setting as “1”and the OFF setting as “0”.

As illustrated, the supply air temperature on graph 601 variesconsiderably between 0.3° C. and −3.5° C. while the return airtemperature varies considerably less between about between 0.2° C. and−0.2° C. The low variability of the return air temperature is caused bythermal inertia of the produce in the transport volume 45 (of FIG. 1).The adjusted setpoint temperature, Tset_quest (long dotted line), iskept constant at about −1.5° C.

By comparing the value of the supply air temperature on graph 601 andthe ON periods of the cooling unit on graph 605, the sudden drop orincrease of supply air temperature in response to switching the coolingunit between Off and ON states is evident.

By inspection of the TEI curve on graph 603 and the ON periods of thecooling unit on graph 605, it is indicated how a TEI value above the 90°C.*min upper bound on the TEI, TEI_max_cool, leads to activation of thecooling unit which stays ON until the flow of cooled supply air hascaused the TEI to drop below the upper bound (and the minimum ON timehas passed). Consequently, the present temperature control algorithmdoes not activate the cooling unit based on certain preset limits orbounds on the supply air temperature or return air temperature butinstead activates the cooling unit based on limits or constraints placedon the TEI.

The activation of the cooling unit on graph 605 also shows that theoperational state of internal fans remains High (state “2”) for theentire depicted time period while the heating unit remains OFF asexpected in view of the high net heat load. The fan speed remains Highduring this short circulation period because the duration of thecirculation period is shorter than the circulation time thresholdt_(ct).

FIG. 7 is series of graphs 701, 703 and 705 illustrating experimentallyrecorded values of selected variables of the above-described temperaturecontrol algorithm under low net heat load conditions of the refrigeratedcontainer. The individual curves of these graphs correspond to those ofFIG. 6. The main difference to the graphs of FIG. 6 is the length of thecirculation periods, i.e. time periods where both the heating unit andthe cooling unit are in OFF state, as evidenced by the relative scalingof the time axes and the curves of graphs 605 and 705 which indicaterespective operational states of the heating unit, cooling unit andinternal fans under the different net head load conditions. During thesecirculation periods in FIG. 7, the internal fans speed starts cyclingbetween Low and Off according to the earlier described rules forcontrolling the fan speed during circulation periods. Time periods wherethe operational state of the cooling unit is ON are always accompaniedby a High speed setting of the internal fans in the present embodimentof the invention.

FIG. 8 is series of graphs 801, 803 and 805 illustrating experimentallyrecorded values of selected variables of the above-described temperaturecontrol algorithm under environmental conditions where heating isrequired to maintain the commodity load at or close to the adjustedsetpoint temperature. Consequently, the coherent nature of the presenttemperature control algorithm capable of supplying heated or cooledsupply air as required is demonstrated. As indicated by curve 813 ongraph 803, a second or lower integral error threshold is set to a value−10° C.*minutes. When the TEI curve 809 falls below this lower integralerror threshold 813, the heating unit is switched ON as depicted ongraph 805 which as expected leads to increasing supply air temperatureas evidenced by the supply air temperature curve 807 of graph 801. Asshown on graph 803, the increasing supply air temperature starting atabout t=9 minutes leads to an increasing value of the TEI after a shortdelay period. The internal fans speed is maintained High during theentire time period while the operational state of the cooling unit isOFF for the entire time period since no cooling is required.

FIG. 9 is a graph illustrating experimentally recorded values of producetemperature, on curve 901, versus supply air temperature, depicted oncurve 903, measured at the same position in the transport volume of therefrigerated container where the supply air enters. It is evident thatproduce temperature is kept within very tight limits of about +/−0.1° C.despite considerable variation of the supply air temperature from about1.5 to 6.4° C. Thus, demonstrating that the thermal inertia of theproduce suffices to annihilate air temperature fluctuations of thisfrequency. The present temperature control system exploits this thermalinertia to maintain highly accurate control over produce temperature bycontrolling the temperature-error integral so as to stay within theupper integral error threshold and the lower integral error threshold.

1. A method of controlling air temperature within a refrigerated storagespace, the method comprising steps of: circulating return air from therefrigerated storage space through a cooling unit to provide a flow ofcooled supply air at a supply air temperature, discharging the flow ofcooled supply air into the refrigerated storage space to control airtemperature inside the refrigerated storage space, measuring the supplyair temperature by a first temperature sensor, computing atemperature-error integral of the supply air based on a difference overtime between the supply air temperature and a reference temperature,adjusting the supply air temperature based on the temperature-errorintegral such that a time average of the supply air temperaturesubstantially equals the reference temperature.
 2. A method ofcontrolling air temperature according to claim 1, wherein the referencetemperature comprises an adjusted setpoint temperature or a setpointtemperature.
 3. A method of controlling air temperature according toclaim 1, comprising a further step of: circulating the return air fromthe refrigerated storage space through a heating unit to provide a flowof heated supply air at the supply air temperature.
 4. A method ofcontrolling air temperature according to claim 1, comprising a step of:controlling respective operational states of the heating unit or thecooling unit or both to adjust the supply air temperature.
 5. A methodof controlling air temperature according to claim 1, comprising a stepof: switching operational states of the cooling unit between exclusivelyON and OFF states to make the adjustment of the supply air temperature.6. A method of controlling air temperature according to claim 5,comprising a further step of: regulating the cooling capacity of thecooling unit by a capacity regulating device coupled to vapourcompression refrigeration cycle, containing a compressor and anevaporator.
 7. A method of controlling air temperature according toclaim 6, wherein the cooling capacity of the cooling unit is regulatedby a step of: controlling a refrigerant flow rate between the evaporatorand the compressor by a suction valve.
 8. A method of controlling airtemperature according to claim 1, comprising further steps of: comparingthe temperature-error integral of the supply air with a first integralerror threshold and a second integral error threshold, changing anoperational state of at least one of the cooling unit, the heating unitand internal fans if the temperature-error integral exceeds the firstintegral error threshold or falls below the second integral errorthreshold.
 9. A method of controlling air temperature according to claim8, wherein a difference between the first integral error threshold andthe second integral error threshold is set to a value between 20°C.*minutes and 200° C.*minutes.
 10. A method of controlling airtemperature according to claim 8, comprising further steps of: settingthe operational state of the cooling unit to active for at least a firstminimum time period if the temperature-error integral exceeds the firstintegral error threshold, setting the operational state of the heatingunit to active for at least a second minimum time period if thetemperature-error integral is below the second integral error threshold.11. A method of controlling air temperature according to claim 8,wherein: the first integral error threshold is set to a value between 50and 200° C.*minutes; and the second integral error threshold is set to avalue between −100 and −10° C.*minutes.
 12. A method of controlling airtemperature according to claim 1, comprising a step of: adjusting thereference temperature as a function of a setpoint temperature and atemperature of the return air measured by a return air temperaturesensor.
 13. A method of controlling air temperature according to claim12, wherein the adjustment of the reference temperature is made suchthat the average of the supply air temperature and the return airtemperature substantially equals the setpoint temperature.
 14. A methodof controlling air temperature according to claim 12, comprising a stepof: limiting the reference temperature to a lower temperature limitdependent on the setpoint temperature, and optionally a further step of:limiting the reference temperature to an upper temperature limit.
 15. Amethod of controlling air temperature according to claim 8, comprisingsteps of, during circulation periods, comparing the temperature-errorintegral to a first heating threshold, maintain internal fans at a firstpreset speed if the temperature-error integral is below the firstheating threshold.
 16. A method of controlling air temperature accordingto claim 15, comprising a further step of: comparing thetemperature-error integral to a second heating threshold smaller thanthe first heating threshold, maintain the internal fans at a secondpreset speed if the temperature-error integral falls below the secondheating threshold; wherein the a second preset speed is higher than thefirst preset speed.
 17. A method of controlling air temperatureaccording to claim 5, comprising further steps of: comparing a durationof a previous circulation period with a circulation time thresholdt_(ct), maintain the speed of the internal fans at a maximum speedduring a current circulation period if the previous circulation periodwas smaller than the circulation time threshold t_(ct).
 18. A method ofcontrolling air temperature according to claim 17, wherein thecirculation time threshold t_(ct) depends on a change of thetemperature-error integral during the previous circulation period.
 19. Amethod of controlling air temperature according to claim 1, wherein thetemperature-error integral is computed at regular time intervals such astime intervals smaller than 1 minute, preferably smaller than 10 secondssuch as about every second.
 20. A method of controlling air temperatureand relative humidity according to claim 1, comprising further steps of:simultaneous heating and cooling the supply air at a first speed settingof the internal fans when a measured relative humidity (RH) of the airinside the transport volume is higher than a first humidity thresholdderived from the setpoint value of the relative humidity (RH_(set)),setting a second fan speed of the internal fans during circulationperiods when a measured relative humidity (RH) of the air is higher thana second humidity threshold derived from the setpoint value of therelative humidity; wherein fan speed in the first fan speed is lowerthan fan speed at the second fan speed setting.
 21. A method ofcontrolling air temperature and relative humidity according to claim 20,comprising further steps: setting the operational state of the heatingunit to ON during circulation periods when a measured relative humidity(RH) of the air is higher than a third humidity threshold derived fromthe setpoint value of the relative humidity.
 22. A refrigerated storagespace comprising: a refrigerated volume for housing a commodity load, acooling unit configured to receive return air from the refrigeratedvolume and generate a flow of cooled supply air at a supply airtemperature, an air flow passage coupled to the refrigerated volume todischarge the supply air therein and control air temperature within therefrigerated volume, a first temperature sensor adapted to measuring thesupply air temperature, a temperature control system adapted to:computing a temperature-error integral of the supply air based on adifference over time between the supply air temperature and a referencetemperature, adjusting the supply air temperature based on thetemperature-error integral such that a time average of the supply airtemperature is substantially equal to the reference temperature.
 23. Arefrigerated storage space according to claim 22, wherein the referencetemperature comprises an adjusted setpoint temperature or a setpointtemperature.
 24. A refrigerated storage space according to claim 22,comprising a heating unit configured to supply a flow of heated supplyair at the supply air temperature from the return air from therefrigerated storage space.
 25. A refrigerated storage space accordingto claim 24, wherein the temperature control system is adapted to:controlling respective operational states of the heating unit or thecooling unit or both to adjust the supply air temperature.
 26. Arefrigerated storage space according to claim 25, wherein thetemperature control system is adapted to: switching operational statesof the cooling unit between exclusively ON and OFF states to make theadjustment of the supply air temperature.
 27. A refrigerated storagespace according to claim 22, wherein the temperature control system isadapted to: adjusting the reference temperature as a function of asetpoint temperature and a temperature of the return air measured by areturn air temperature sensor.
 28. A refrigerated storage spaceaccording to claim 27, wherein the temperature control system is adaptedto adjust the reference temperature such that the average of the supplyair temperature and the return air temperature substantially equals thesetpoint temperature.
 29. A refrigerated storage space according toclaim 22, comprising a refrigerated container wherein the refrigeratedvolume comprises a transport volume of the refrigerated container.